I currently have an hp d4100y desktop with a pentium D processor. I am curious if its possible for me to purchase a core 2 duo processor and swap it in, in place of the pentium D. I'm just curious as to whether the motherboard I have is compatible with the core 2 processor and if it would just be a situation of drop it in and go. Thanks ahead of time for any help.

Doubtful. Your motherboard has a Intel 945P chipset. While some 945P mobos on NewEgg support C2D, many don't. The only BIOS update listed for your model does not mention C2D, only "newer Intel Processors." Since it is dated May '06, i doubt it includes C2D.